YeonGyu-Kim 2678fa0af5 fix: #124 --model validation rejects malformed syntax at parse time
Adds validate_model_syntax() that rejects:
- Empty strings
- Strings with spaces (e.g., 'bad model')
- Invalid provider/model format

Accepts:
- Known aliases (opus, sonnet, haiku)
- Valid provider/model format (provider/model)

Wired into parse_args for both --model <value> and --model=<value> forms.
Errors exit with clear message before any API calls (no token burn).

Verified:
- 'claw --model "bad model" version' → error, exit 1
- 'claw --model "" version' → error, exit 1
- 'claw --model opus version' → works
- 'claw --model anthropic/claude-opus-4-6 version' → works

Refs: ROADMAP #124 (debbcbe cluster — parser-level trust gap family)
2026-04-20 16:32:17 +09:00
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